TOF (acronym of “Time Of Flight”), LIDAR (acronym of “Light detection and ranging”) or LADAR (acronym of “Laser detection and ranging”) measurement systems are systems which allow measuring distances by means of using a light source illuminating the points being measured.
The measurable distance in TOF systems is influenced by some uncontrolled factors which depend on the environment and not on the TOF technique, such as for example, the background illumination intensity, the weather conditions (fog, dust, rain, etc.) or the reflectance of the object.
In addition, there are other aspects that depend directly on the technology and the architecture of the TOF device and play a main role in the determination of the measurable distance of each system. Among said aspects, the most significant aspects could be the illumination source power, light beam divergence, point scanning system efficiency, photodetector sensitivity, optical system attenuation or background light filtering quality.
The physical principles of TOF, LIDAR or LADAR systems state that, expressed in a very general manner, the capability of measuring a point located at a certain distance is related to the capability of illuminating it with sufficient optical power to detect the light beam reflected thereon in a light detector. This principle has a decisive influence on the measurable distance of the device and different scanning techniques have been designed based on same in order to take measurements of TOF images. There are TOF devices which can measure distances up to dozens of kilometers and others are limited to a few meters. The illuminating beam in far-reaching systems usually has little divergence. These tend to use highly collimated laser light beams with a very small divergence and beam section. With this, the energy concentration per unit of surface in the object is much greater if compared with systems using diverging light sources. Systems using diverging light sources illuminate larger areas for simultaneously measuring a set of points instead of only one.
To take measurements of three-dimensional image by means of TOF, LIDAR or LADAR techniques, it is necessary to measure a set of points forming a three-dimensional image (or point cloud) and to that end, it is necessary to illuminate the surface corresponding to the image to be measured. The technologies which allow measuring a set of points in a controlled manner for forming three-dimensional images are basically divided into two:                Sequential scanning systems.        Systems based on arrays of detectors.        
The sequential scanning systems form the 3D image by sequentially measuring unique points. The illumination sequence is usually implemented by means of optical systems such as galvanometric mirrors, MEMS, acoustic optical deflectors, etc. . . . . Given a specific optical power, the sequential scanning systems concentrate said power at a small point measuring greater distances in comparison with beam-expanding systems. The collimation and the small size of the illuminated point mean that energy concentration per unit of surface is higher than in the case of diverging light sources. This high energy concentration per unit of surface means that the light reflected by the object is greater and as a result, the detector receiving the light from that point also receives a larger amount of light. Taking into account that one of the main factors that sets the limitation in the capability of measuring in distance in LIDAR systems is the capability of detecting low power reflected optical beams, the effect of energy concentration of the sequential scanning systems successfully maximizes the measurable distance as a result of harnessing all the optical power available for a single point of measurement. The greater the energy concentration per unit of surface, more energy flow is reflected on the point of measurement and accordingly, the easier the detection is. Although the sequential scanning systems allow obtaining a high spatial resolution in the three-dimensional image, performing point-to-point measurement means that the total measurement time is high in order to attain images having a high spatial resolution. This limits the amount of images which can be measured per second. By way of example, commercial equipment based on this technology such as Riegl laser scanners (http://www.riegl.com/), MDL laser scanners (http://www.mdl-laser.com) or Faro laser scanners (http://www.faro.com), can be mentioned.
In addition, there are systems based on arrays of detectors. These systems use a set of detectors arranged in grid in which each of them has the TOF measuring capability, based on pulsed TOF, TOF-FMCW or TOF-“Range Gated”. In these systems, the points of the three-dimensional image are measured simultaneously through a set of detectors. In other words, said detectors measure in a synchronized manner a set of distance values equivalent to the number of detectors of the array and, at the same time, that set of measured points forms a three-dimensional image. The main advantage of these systems lies in the fact that several measurements are performed simultaneously which allows measuring a complete image through a single TOF measuring action. For example, in the case of pulsed TOF, a complete three-dimensional image can be measured through a single laser pulse (nevertheless, some equipment use more than one pulse to increase the quality of the image through successive measurement integration).
A condition necessary for performing simultaneous measurements through an array of detectors (partially or completely, i.e., using the entire array or only a part thereof) involves simultaneously illuminating all those points in the object which will be measured by the group of detectors. This means that the optical power is divided among all the points. Therefore, given an illumination source having a specific energy, the energy concentration per unit of surface will be distributed among all the points of measurement causing the energy reflected by each individual point to be inversely proportional to the number of measured points. By way of example, the “Flash LADAR” systems of the company Advanced Scientific Concepts (http://www.advancedscientificconcepts.com/), the pulsed TOF cameras of Odos Imaging (http://odos-imaqing.com/), as well as most TOF cameras based on TOF-FMCW (Time-of-Flight Frequency Modulated Continuous Waveform) measurement, for example, cameras from Mesa Imaging (www.mesa-imaging.ch), PMD (www.pmdtec.com) or SoftKinetic (http://www.softkinetic.com), can be mentioned.
The main advantage of this system with respect to the sequential scanning systems lies in the fact that the points are measured in parallel successfully measuring a large number of points for each TOF measuring action. In contrast, the sequential scanning systems perform a single measurement per TOF measuring action. Generally, it can be stated that considering an illumination source having determined power and shared between the two systems, the amount of three-dimensional images measured will be greater for the systems based on arrays of detectors than in the sequential measurement systems as a result of the effect of parallel measurements. Nevertheless, given that the illumination energy is finite and, in this comparative case, the same between the two systems, the measured distance will be greater in the sequential scanning system than in the of array of detectors given that the energy used for the point of measurement will be greater because all the power of the source is concentrated on one and the same point of measurement.
At an intermediate point, there are pieces of equipment which are made up of sets of detectors performing measurements simultaneously and sequential scanning at the same time. This technique is usually used for measuring larger surfaces. By way of example, the system of the company Velodyne Lidar (http://velodynelidar.com) can be highlighted. This system performs simultaneous measurements through a set of detectors while at the same time a rotary mechanical head performs scanning circularly in a 360° angle to attain a circular field of view. It can be considered that such systems bring together the characteristics of the two general methods described above since they are capable of measuring a set of points simultaneously while at the same time performing sequential scanning to measure all the points forming the final three-dimensional image.
In most systems based on array of detectors, the spatial resolution of the three-dimensional image is fixed by the number of detectors of the array of detectors. Nevertheless, there is a system which allows obtaining a spatial resolution in the image greater than that of the array of detectors. Said TOF system is described in the international patent application WO2012123809A1, and allows increasing the spatial resolution of the three-dimensional image as a result of the inclusion and use of light switches arranged in grid or an array of light switches (such as a light spatial modulator based on micromirrors, such as the case of a DMD: Digital Micromirror Device), in a number greater than light detectors, sequentially redirecting towards the array of light detectors the different portions of light reflected on the surface to be scanned. The inventors refer to that technology as “Digital Scanning” and it is considered to be located in an intermediate level between systems based on array of detectors and sequential scanning as it implements the two methods. Nevertheless, said scanning is digitally controlled and moving parts are not involved in same.
One of the uses of the system described in international patent application WO2012123809A1 is the measurement of three-dimensional images by means of the TOF or time of flight technique. A light source illuminates the surface to be measured. The DMD receives said beam through an optical group and sequentially redirects the received beam towards a detector or an array of detectors having TOF measuring capability. Given that the DMD is optically conjugated with the object being measured, it is capable of receiving in a controlled manner the light from each point of measurement. Each light switch is conjugated with a point of the surface to be measured, therefore, each light switch is capable of directing said portion of the total beam to the detector system or of rejecting same. Given that the DMD or array of switches and the detector or array of detectors are optically conjugated, the DMD is capable of directing said portions of the beam, which at the same time correspond to the light reflected by the points of the object to be measured, to the set of detectors in a controlled manner. Through a sequential process, the DMD will receive and direct the light reflected on the object towards the detector or group of detectors such that, the DMD will simultaneously send as many portions of the beam as the number of detectors present in said array of TOF detectors. In other words, a number of simultaneous TOF measurements equivalent to the number of detectors will be performed. The sequential measurement process consists of directing in a controlled manner all the points of the surface which the DMD is optically configured to receive. The resulting three-dimensional image will have as many measured points as the number of light switches in the DMD. Considering that the DMD has a much larger number of light switches than the array of detectors, the resulting image will have a spatial resolution greater than the number of detectors. This characteristic means that through a small group of detectors, and in turn less complex technically and more cost-effective than one with a large amount of detectors, TOF images having a high spatial resolution and with added functionalities can be measured.
One of the characteristics of this system is that for each TOF measuring action it is necessary to illuminate the entire surface to be measured. The entire surface is understood as the entire set of points which will form the three-dimensional image resulting from performing the sequential measurement process. Like what occurs in the systems based on arrays of detectors described above, this system illuminates the entire surface to be scanned causing the optical power available to be distributed among all the points of the surface in each TOF measuring action and only a small group of points (equivalent to the number of detectors) will be measured. The larger the number of detectors in the array of detectors, the more illumination energy is harnessed given that the number of simultaneously measured points is greater and the number of rejected points is lower. This has an impact on the light energy received in each point of the surface and, accordingly, negatively affects the distance of detection given that the illumination energy is divided among all the points of the surface.
The TOF three-dimensional image measurement systems have several applications and markets in which these systems are of use. Only by way of example, an application of interest for which it would be of interest to use such systems belongs to the field of automotive industry, particularly systems for monitoring, detecting and recognizing objects in the environment of an automobile in order to obtain information about the space in which this automobile circulates for purposes of safety, navigation or artificial intelligence while driving.
For such application, there are various fundamental performance requirements that must be met by the system to assure the use thereof in said context. Some of them are specified below only by way of example:                Operation in an outdoor environment under conditions with a large amount of daylight and background illumination.        Measurable distance up to 100 meters.        Viewing angle: horizontal ±20°, vertical ±5°.        Real time image measurement (>10 Hz).        
Considering a distance of 100 m with said viewing angles, the surface to be measured has about 750 m2. A TOF system based on an array of detectors will have to completely illuminate such surface in each TOF measuring action. Taking into account the radiometric parameters involved in the illumination, reflection and detection process of said beam, also considering the amount of background light and the detector parameters, it can be concluded that the illumination energy necessary for being able to be detected in the array of detectors is potentially very high. A sequential scanning system would require less illumination energy but its performance in terms of measurement speed would also be limited, being able to have limitations in moving object measurement.
Additionally, the inverse-square law establishes that the light intensity on a surface receiving light from a point light source is inversely proportional to the square of the distance between the light source and the surface and proportional to the cosine between the light beam and the normal to the surface. This means that the illumination intensity on a determined area will decrease with distance according to a quadratic factor. When a surface which is illuminated with a light source is moved away from the light source, the illumination intensity of the surface decreases, the illumination intensity decreasing much faster than the surface is being moved away from the light source. For example, if the illumination on a surface is 40 lux at a distance of 0.5 m from the light source, the illumination decreases to 10 lux at a distance of 1 m. This phenomenon decisively influences the measurable distance in a TOF system. In systems based on arrays of detectors, this effect can be acceptable for short distances (10 to 15 m) where the illumination intensity per m2 remains high, but when measurement of medium-long distances (more than 15 m) and large areas is required, this phenomenon becomes a problem, since the illumination sources have limited energy. According to the knowledge of the present inventors, this is a real limiting factor in terms of distance measured in systems based on arrays of detectors.
To that end, and based on a series of studies based on simulations of different radiometric models performed by the present inventors, it can be said that use of systems based on arrays of detectors for applications in the field of automotive industry, in which measurements of at least 100 meters are required, is clearly unviable, because it would involve using a laser source having enormous power, which are very expensive, have a high consumption and are incompatible with the safety rules for eyes. The aforementioned commercial TOF cameras work well for certain applications (indoor environments and for ranges of short distances) but have serious limitations in outdoor environments with daylight and for medium-long distances. It must be noted that most of them use LEDs as a light source the power of which is substantially less compared with the laser sources used in sequential scanning systems.
The foregoing can be extrapolated to many other fields of application different from the field of automotive industry, all of them under the mentioned influence of the inverse-square law, although each field of application will have its particular restrictions relating to operating environment, measurement distances, viewing angles, etc.
According to the knowledge of the inventors, there is no TOF device today which meets the requirements herein described for being applied on a massive scale in the field of automotive industry, even meeting the price requirements.
For such application in the field of automotive industry, and for many other applications of interest, it would be of interest to provide a system combining the advantages of the two methods for generating 3D images in TOF, i.e., the advantages of sequential scanning and the advantages of the systems based on arrays of detectors. The objective thereof would be to perform measurements on objects located at a greater distance than that covered by the systems based on arrays of detectors, with a good spatial resolution, a measurement speed greater than the sequential scanning systems and, using light sources having less power.
Patent application US20120249999A1 discloses one of such combined systems, since it proposes combining a “Flash LADAR” system with a laser scanning system for the purpose of using lasers with less power if the complete field of view does not have to be measured. In this system, the “Flash LADAR” component measures the distance to the illuminated object by means of TOF and the scanning system selectively illuminates said object. The inventors describe a series of applications such as the detection and tracking of stationary and/or moving objects, navigation or collision avoidance systems always based on the “Flash LADAR” technology also patented by the same inventors.
By means of the system proposed by patent document US20120249999A1, a laser light beam is projected on a sub-area (object) to be detected contained within the field of view, with a determined divergence so that a simultaneous measurement of said entire sub-area, including a single pixel or a small group of pixels, takes place, i.e., using a divergence greater than that of sequential scanning systems and less than that of systems based on array of detectors.
To direct the laser towards the sub-area to be scanned, a mirror of a galvanometric system (for example, MEM type) is used, so it can be said that the system of US20120249999A1 is actually a combination of the two TOF systems described above.
It is indicated that in the system proposed in US20120249999A1, the illumination beam can be varied so that it illuminates the field of view of the Flash LADAR system entirely (all the pixels of the array of detectors) or only partially (one or more pixels of the array of detectors) depending on the application.
In the system of patent document US20120249999A1 detection is performed with an array of light detectors the total resolution of which is adapted to the total area of the surface or scene to be scanned, so when they illuminate the mentioned sub-area a lower spatial resolution is obtained, i.e., if they only illuminate 10% of the total area, only 10% of the pixels of the array of light detectors will be illuminated, i.e., will receive reflected light, so a spatial resolution of only 10% of the total resolution of the array will be obtained, which means that such system provides rather poor results in terms of spatial resolution. In other words, the detectors of the array which are optically conjugated with the pixels of the sub-areas which are not being illuminated cannot be used for TOF measurement, causing this underuse of the set of detectors of the array to negatively affect the spatial resolution of the TOF image in comparison with the case of using a light source completely illuminating the field of view of the array of detectors in which all the detectors are used.